Polypropylene compositions have gained wide commercial acceptance and usage in numerous applications because of the relatively low cost of the polymers and the desirable properties they exhibit. In general, polypropylene polymers, particularly propylene homopolymers, have a disadvantage of being brittle with low impact resistance, especially at low temperatures. To address these issues, manufacturers have incorporated a dispersed copolymer phase (often called a “rubber” phase), which forms a dispersed phase within the polypropylene matrix. These two-phase materials are referred to as impact copolymers or ICPs.
While impact resistance is improved, a major drawback to such materials is the poor transparency, mostly due to the rubber particles being large enough to affect light transmission or a refractive index mismatch in the heterophasic system. Accordingly, several attempts have been made to improve the transparency of heterophasic polymer systems.
Some ICPs have been able to achieve clarity by refractive index matching the dispersed phase to the matrix material. Other ICPs have sought to achieve clarity by increasing dispersed phase miscibility to achieve particles which are not large enough to affect the light transmission. Known ICPs generally do not provide a combination of good properties including stiffness, toughness and optical transparency.